1. Field of the Invention
The present invention relates to the assembly of a faceplate with a baseplate as used in field emission displays ("FEDs") and plasma displays. More specifically, it relates to a process for accurately maintaining an alignment of the faceplate with the baseplate in preparation for, and throughout, the sealing process.
2. Description of the Related Art
A FED, as is well known in the art, is a flat panel display which has a faceplate, on which phosphor pixels reside, and a plurality of micro-tip cathode members spaced a short distance from the faceplate, which emit electrons to activate the phosphors. In some embodiments, each cathode member is attached to, or is integrally formed with, a backplate. In other embodiments, each cathode member is also attached to the faceplate and all of the cathode members are surrounded by a separate backplate.
Similarly, plasma displays, as is well known in the art, are flat panel displays that, in the simplest configuration, consist of two glass plates; each with parallel electrodes deposited on their surfaces. The plates are assembled with their electrodes at right angles, and the gap between the plates is filled with a rare gas mixture. Each pixel at the intersection of a line and a column electrode can be illuminated independently when a voltage pulse is applied between the two electrodes. The voltage pulse leads to the breakdown of the gas and to the formation of a weakly ionized plasma that emits visible or UV light.
For both FEDs and plasma displays, it is necessary to accurately align the faceplate and baseplate of the display and then seal the plates together by melting a high temperature (approximately 500.degree. C.) glass frit seal while the plates are pressed together. In order to achieve this, traditionally, an alignment tool is used to control the x and y positions, and the pitch and roll of the plates relative to each other so they stay aligned during the sealing process.
FIGS. 1-3 depict two conventional methods of achieving the above mentioned frit seal. Referring first to FIG. 1, a backside view of a faceplate 20 is shown. The glass frit 22 is usually placed around all of the edges of either glass plate (i.e., baseplate or faceplate), with FIG. 1 showing the glass frit 22 around the edges of the backside of faceplate 20. Spacers 23 are usually located at spaced apart locations on the faceplate and provide for the necessary final spatial gap between the front and base plates. Since the gap will either be a vacuum or filled with a rare gas mixture, a hole may be left in the seal 22 for purposes of attaching an exhaust tube.
Referring now to FIG. 2(a), a side view of a typical sealing structure (as in FIG. 1) is shown. Baseplate 21 is depicted as being in alignment with faceplate 20. Glass frit 22 has been disposed around faceplate 20 and spacers 23a, 23b provide the inherent gap. After alignment, baseplate 21 is moved in the direction A of faceplate 20 until baseplate 21 is touching the glass frit 22, as seen in FIG. 2(b).
When the baseplate 21 makes contact with the glass frit 22, as seen in FIG. 2(b), the FIG. 2(b) assembly is heated to temperatures upwards of 500.degree. C. until the glass frit 22 begins to soften. With the glass frit 22 softening due to the increased temperature, the plates 20, 21 are moved closer together through the use of clamps until the excess glass frit 22 is extruded slightly between the plates, as depicted in FIG. 2(c). During this assembly process, however, there is no way of assuring the plates 20, 21 will remain in accurate alignment, unless some alignment mechanism is employed to maintain alignment.
FIG. 3(a) depicts an alternative method for sealing the face and baseplates which is also known in the art and is described in, for example, U.S. Pat. No. 5,807,154, to Watkins, incorporated herein by reference. In this method, baseplate 20 and faceplate 21 are in alignment. Spacers 23a, 23b, are located at the outer edges of the baseplate 20. Each spacer 23a, 23b supports a small deposit of an adhesive material 33, and also a small deposit of glass frit 32. When the faceplate 21 is touching the adhesive material 33, the faceplate 21 is relatively stationary and the assembly is ready to be heated. In other words, the adhesive 33 holds plates 20, 21 in an aligned condition. The plates 20, 21 are then pressed further together as the heat is applied until the plates are in the FIG. 3(b) sealed configuration.
During the early stages of the sealing process, the adhesive material 33 is removed due to the applied heat, leaving only the glass frit 32 behind to form an hermetic seal between the plates 20, 21. While this method is useful and solves some alignment problems, there is no sure way of maintaining the plates 20, 21 in alignment during the entire assembly process without the use of a mechanical fixture. This is especially true during the initial heating stages in which the adhesive is melted from between the plates. In such a situation, it is not uncommon, absent use of an alignment tool, to have an inadvertent shifting of the plates.